Method of manufacturing and using rod-shaped proppants and anti-flowback additives

ABSTRACT

A sintered rod-shaped proppant and anti-flowback agent possesses high strength and high conductivity. The sintered rods comprise between about 0.2% by weight and about 4% by weight aluminum titanate. In some embodiments, the sintered rods are made by mixing bauxitic and non-bauxitic sources of alumina that may also contain several so-called impurities (such as TiO 2 ), extruding the mixture, and sintering it. The starting material may optionally be milled to achieve better compacity and crush resistance in the final sintered rod. A fracturing fluid may comprise the sintered rods alone or in combination with a proppant, preferably a proppant of a different shape.

FIELD OF THE INVENTION

The present invention relates to methods of making and using proppantsfor fractured earth having a high compressive strength andsimultaneously a good conductivity. It also relates to methods of makingand using anti-flowback additives for use in fracturing operations.

BACKGROUND

Naturally occurring deposits containing oil and natural gas have beenlocated throughout the world. Given the porous and permeable nature ofthe subterranean structure, it is possible to bore into the earth andset up a well where oil and natural gas are pumped out of the deposit.These wells are large, costly structures that are typically fixed at onelocation. As is often the case, a well may initially be very productive,with the oil and natural gas being pumpable with relative ease. As theoil or natural gas near the well bore is removed from the deposit, otheroil and natural gas may flow to the area near the well bore so that itmay be pumped as well. However, as a well ages, and sometimes merely asa consequence of the subterranean geology surrounding the well bore, themore remote oil and natural gas may have difficulty flowing to the wellbore, thereby reducing the productivity of the well.

To address this problem and to increase the flow of oil and natural gasto the well bore, companies have employed the well-known technique offracturing the subterranean area around the well to create more pathsfor the oil and natural gas to flow toward the well. As described inmore detail in the literature, this fracturing is accomplished byhydraulically injecting a fluid at very high pressure into the areasurrounding the well bore. This fluid must then be removed from thefracture to the extent possible to ensure that it does not impede theflow of oil or natural gas back to the well bore. Once the fluid isremoved, the fractures have a tendency to collapse due to the highcompaction pressures experienced at well-depths, which can be more than20,000 feet. To prevent the fractures from closing, it is well-known toinclude a propping agent, also known as a proppant, in the fracturingfluid. The goal is to be able to remove as much of the injection fluidas possible while leaving the proppant behind to keep the fracturesopen. As used in this application, the term “proppant” refers to anynon-liquid material that is present in a proppant pack and providesstructural support in a propped fracture. “Anti-flowback additive”refers to any material that is present in a proppant pack and reducesthe flowback of proppant particles but still allows for production ofoil at sufficient rates. The terms “proppant” and “anti-flowbackadditive” are not necessarily mutually exclusive, so a single particletype may meet both definitions. For example, a particle may providestructural support in a fracture and it may also be shaped to haveanti-flowback properties, allowing it to meet both definitions.

Several properties affect the desirability of a proppant. For example,for use in deep wells or wells whose formation forces are high,proppants must be capable of withstanding high compressive forces, oftengreater than 10,000 pounds per square inch (“psi”). Proppants able towithstand these forces (e.g., up to and greater than 10,000 psi) arereferred to as high strength proppants. If forces in a fracture are toohigh for a given proppant, the proppant will crush and collapse, andthen no longer have a sufficient permeability to allow the proper flowof oil or natural gas. Other applications, such as for use in shallowerwells, do not demand the same strength proppant, allowing intermediatestrength proppants to suffice. These intermediate strength proppants aretypically used where the compressive forces are between 5,000 and 10,000psi. Still other proppants can be used for applications where thecompressive forces are low. For example, sand is often used as aproppant at low compressive forces.

In addition to the strength of the proppant, one must consider how theproppant will pack with other proppant particles and the surroundinggeological features, as the nature of the packing can impact the flow ofthe oil and natural gas through the fractures. For example, if theproppant particles become too tightly packed, they may actually inhibitthe flow of the oil or natural gas rather than increase it.

The nature of the packing also has an effect on the overall turbulencegenerated through the fractures. Too much turbulence can increase theflowback of the proppant particles from the fractures toward the wellbore. This may undesirably decrease the flow of oil and natural gas,contaminate the well, cause abrasion to the equipment in the well, andincrease the production cost as the proppants that flow back toward thewell must be removed from the oil and gas.

The useful life of the well may also be shortened if the proppantparticles break down. For this reason, proppants have conventionallybeen designed to minimize breaking. For example, U.S. Pat. No. 3,497,008to Graham et al. discloses a preferred proppant composition of a hardglass that has decreased surface flaws to prevent failure at thoseflaws. It also discloses that the hard glass should have a goodresistance to impact abrasion, which serves to prevent surface flawsfrom occurring in the first place. These features have conventionallybeen deemed necessary to avoid breaking, which creates undesirable fineswithin the fracture.

The shape of the proppant has a significant impact on how it packs withother proppant particles and the surrounding area. Thus, the shape ofthe proppant can significantly alter the permeability and conductivityof a proppant pack in a fracture. Different shapes of the same materialoffer different strengths and resistance to closure stress. It isdesirable to engineer the shape of the proppant to provide high strengthand a packing tendency that will increase the flow of oil or naturalgas. The optimum shape may differ for different depths, closurestresses, geologies of the surrounding earth, and materials to beextracted.

The conventional wisdom in the industry is that spherical pellets ofuniform size are the most effective proppant body shape to maximize thepermeability of the fracture. See, e.g., U.S. Pat. No. 6,753,299 toLunghofer et al. Indeed, the American Petroleum Institute's (“API's”)description of the proppant qualification process has a sectiondedicated to the evaluation of roundness and sphericity as measured onthe Krumbein scale. However, other shapes have been suggested in theart. For example, previously-mentioned U.S. Pat. No. 3,497,008 to Grahamet al. discloses the use of “particles having linear, parallel, oppositesurface elements including cylinders, rods, paralellepipeds, prisms,cubes, plates, and various other solids of both regular and irregularconfigurations.” (Col. 3, lines 34-37.) According to that patent, thedisclosed shape configuration has several advantages when used as aproppant, including increased conductivity over spherical particles(col. 4, lines 29-35), greater load bearing capacity for the samediameter as a spherical particle (col. 4, lines 36-38), a higherresistance to being embedded in the fracture wall (col. 4, lines 45-47),and a lower settling rate (col. 4, lines 58-60).

Despite this disclosure of the potential advantages of using rod-likeparticles for proppants, the industry had not embraced the suggestion.The applicants are not aware of any rod-like particles on the marketthat are used as proppants or anti-flowback additives. Indeed, morerecent patents cast doubt on the effectiveness of using rod-like shapes.For example, U.S. Pat. No. 6,059,034 to Rickards et al. discloses themixing of rod-like fibrous materials with another proppant material toprevent proppant movement and flowback. According to that patent, “inpractice this method has proven to have several drawbacks, includingreduction in fracture conductivity at effective concentrations offibrous materials, and an effective life of only about two years due toslight solubility of commonly used fiber materials in brine. Inaddition, fiber proppant material used in the technique may beincompatible with some common well-treating acids, such as hydrofluoricacid.” (Col. 2, lines 36-43.) Although the rod-like fibrous materialsare used in conjunction with another proppant, the patent suggests thatrod-like particles in a fracturing fluid are undesirable.

Another property that impacts a proppant's utility is how quickly itsettles both in the injection fluid and once it is in the fracture. Aproppant that quickly settles may not reach the desired proppinglocation in the fracture, resulting in a low level of proppants in thedesired fracture locations, such as high or deep enough in the fractureto maximize the presence of the proppant in the pay zone (i.e., the zonein which oil or natural gas flows back to the well). This can causereduced efficacy of the fracturing operation. Ideally, a proppantdisperses equally throughout all portions of the fracture. Gravity worksagainst this ideal, pulling particles toward the bottom of the fracture.However, proppants with properly engineered densities and shapes may beslow to settle, thereby increasing the functional propped area of thefracture. How quickly a proppant settles is determined in large part byits specific gravity. Engineering the specific gravity of the proppantfor various applications is desirable because an optimized specificgravity allows a proppant user to better place the proppant within thefracture.

Yet another attribute to consider in designing a proppant is itsacid-tolerance, as acids are often used in oil and natural gas wells andmay undesirably alter the properties of the proppant. For example,hydrofluoric acid is commonly used to treat oil wells, making aproppant's resistance to that acid of high importance.

Still another property to consider for a proppant is its surfacetexture. A surface texture that enhances, or at least does not inhibit,the conductivity of the oil or gas through the fractures is desirable.Smoother surfaces offer certain advantages over rough surfaces, such asreduced tool wear and a better conductivity, but porous surfaces maystill be desirable for some applications where a reduced density may beuseful.

All of these properties, some of which can at times conflict with eachother, must be weighed in determining the right proppant for aparticular situation. Because stimulation of a well through fracturingis by far the most expensive operation over the life of the well, onemust also consider the economics. Proppants are typically used in largequantities, making them a large part of the cost.

Attempts have been made to optimize proppants and methods of using them.Suggested materials for proppants include sand, glass beads, ceramicpellets, and portions of walnuts. The preferred material disclosed inpreviously-mentioned U.S. Pat. No. 3,497,008 is a hard glass, but italso mentions that sintered alumina, steatite, and mullite could beused. Conventional belief is that alumina adds strength to a proppant,so many early proppants were made of high-alumina materials, such asbauxite. The strength of these high-alumina materials is believed to bedue to the mechanical properties of the dense ceramic materials therein.See, e.g., U.S. Pat. Nos. 4,068,718 and 4,427,068, both of whichdisclose proppants made with bauxite.

Bauxite is a natural mineral comprising various amounts of four primaryoxides: alumina (Al₂O₃, typically from about 80% to about 90% byweight), silica (SiO₂, typically from about 1% to about 12% by weight),iron oxide (Fe₂O₃, typically from about 1% to about 15% by weight), andtitania (TiO₂, typically from about 1% to about 5% by weight). Aftercalcining or sintering, bauxite is known to have a higher toughness buta lower hardness than technical grade alumina-based ceramics. Sincetoughness is a primary mechanical characteristic to consider inimproving the crush resistance or compressive strength of ceramics,bauxite is of interest for use in proppants. The microstructure ofbauxite is characterized primarily by three phases: 1) a matrix of finealumina crystal; 2) a titania phase where titania is complexed withalumina to form aluminum titanate (Al₂TiO₅); and 3) a mullite phase(3Al₂O₃, 2SiO₂). For the first two phases a partial substitution ofaluminum by iron atoms is possible. To achieve good mechanicalcharacteristics as a proppant, bauxite with lower levels of silica andiron oxide are preferred.

For example, previously-mentioned U.S. Pat. No. 4,427,068 discloses aspherical proppant comprising a clay containing silica that adds aglassy phase to the proppant, thereby weakening the proppant.Furthermore, the silica of that patent is so-called “free” silica. Ingeneral, high amounts of silica reduce the strength of the finalproppant. In particular, it is believed that proppants containing morethan 2% silica by weight will have reduced strength over those withlower silica contents. Other so-called impurities are also believed toreduce the strength of the proppant.

Early high strength proppants were made using tabular alumina which wasa relatively expensive component. For this reason, the industry shiftedfrom using tabular alumina to other alumina sources, such as bauxite. Bythe late 1970's, the development focus in the industry shifted from highstrength proppants to intermediate or lower strength, lower densityproppants that were easier to transport and use, and were lessexpensive. Over the next 20 years, the industry focused oncommercialization of lower density proppants and they became commonlyused. The primary method of reducing the density of proppants is toreplace at least a portion of the higher density alumina with lowerdensity silica. According to U.S. Pat. No. 6,753,299, “the originalbauxite based proppants of the early 1970's contained >80% alumina(Cooke). Subsequent generations of proppants contained an aluminacontent of >70% (Fitzgibbons), 40% to 60% (Lunghofer), and later 30% to<40% (Rumpf, Fitzgibbons).” Thus, as to both product development andproppant use, there was a retreat in the industry from proppantsmanufactured from high-alumina materials such as bauxite.

Today, as resources become more scarce, the search for oil and gasinvolves penetration into deeper geological formations, and the recoveryof the raw materials becomes increasingly difficult. Therefore, there isa need for proppants that have an excellent conductivity andpermeability even under extreme conditions. There is also need forimproved anti-flowback additives that will reduce the cost of productionand increase the useful life of the well.

SUMMARY OF THE INVENTION

A method is provided for making a proppant or anti-flowback additive.The method comprises providing a composition comprising at least about90% by weight alumina and between about 0.15% and about 3.5% by weightTiO₂; milling the composition; forming at least one rod from the milledcomposition; and sintering the at least one rod.

Another method is provided for making a proppant or anti-flowbackadditive. The method comprises forming a rod-shaped article from amilled composition, where the milled composition comprises at leastabout 90% by weight alumina and between about 0.15% and about 3.5% byweight TiO₂. The article is also sintered.

Another method for making a proppant or anti-flowback additive comprisessintering a rod-shaped article made from a milled composition comprisingat least about 90% by weight alumina and between about 0.15% and about3.5% by weight TiO₂.

Another method of making a proppant or anti-flowback additive comprisessintering a rod-shaped article formed from a milled composition, themilled composition comprising at least about 80% technical grade aluminaby weight and between about 0.1% by weight and about 20% by weight ofmaterial containing at least one other oxide.

A method is provided for fracturing subterranean formations. The methodcomprises injecting a fluid containing a sintered rod-shaped articlemade from a milled composition, where the milled composition comprisesat least about 90% by weight alumina and between about 0.15% and about3.5% by weight TiO₂.

Another method of fracturing subterranean formations comprises injectinga fluid containing a sintered rod-shaped article formed from a milledcomposition, the milled composition comprising at least about 80%technical grade alumina by weight and between about 0.1% by weight andabout 20% by weight of material containing at least one other oxide.

The foregoing background and summary are not intended to becomprehensive, but instead serve to help artisans of ordinary skillunderstand the following implementations consistent with the inventionset forth in the appended claims. In addition, the foregoing backgroundand summary are not intended to provide any limitations on the claimedinvention.

DESCRIPTION OF THE INVENTION

Reference will now be made in detail to embodiments of the presentinvention. A high strength proppant and anti-flowback additive having arod shape is found to achieve superior conductivity and other benefitswhen used in hydraulic fracturing of subterranean formations surroundingoil and/or gas wells under relatively high closing pressures.

A high strength proppant in accordance with one embodiment of thepresent invention is a solid rod-shaped particle prepared by sinteringan alumina-containing material, such as, for example, technical gradealumina, bauxite, or any other suitable combination of oxides thereof.The rod-shaped particle may have a solid trunk bounded by twosubstantially parallel planes. In one preferred embodiment of thepresent invention, the two substantially parallel planes may besubstantially circular, thereby creating a cylindrical trunk. Othersuitable shapes may be also be used as the bounding planes. It ispreferable that the bounding plane shapes have a minimum number ofangles, such as circles or ovals or other symmetrical or asymmetricalshapes with rounded edges, such as egg curves, because angular particlestend to pack more tightly together and concentrate the pressure on thecontact points between the particles because of their sharp edges. Thisincreased pressure can lead to an increased likelihood that theproppants will undesirably break into fine particles. Angular shapes,such as triangles, squares, rectangles, etc., where one or more of thecorners is rounded may also be used as the bounding planes withoutdeparting from the spirit of the present invention. The rod bounded bythese different shapes may take on trunks of different shapes, forexample, in the shape of a triangular prism, without departing from thespirit of the present invention.

The sintered rod is found to exhibit superior hardness and toughness. Asknown in the art, increased alumina (Al₂O₃) content in the sinteredproduct results in increased hardness and toughness. Sintered rodsconsistent with one embodiment of the present invention may have a highalumina content, for example, greater than about 80% alumina by weight.In some embodiments, the alumina content may be increased to greaterthan about 90% by weight. It may further be preferable that the aluminacontent be greater than about 92% by weight, with the optimum hardnessand toughness being achieved between about 92% and about 96% alumina byweight.

It has also been found that the presence of aluminum titanate (Al₂TiO₅)in the sintered rod results in improved hardness and toughness. Thesintered rod may contain between about 0.2% and about 4% aluminumtitanate, preferably between about 0.5% and about 3%, and mostpreferably between about 1% and about 2.5%. In one embodiment, thealuminum titanate is formed during sintering when the pre-sinteredmaterial includes a small percentage of TiO₂. The TiO₂ may becontributed by non-bauxitic sources or, preferably, bauxite. In oneembodiment, the pre-sintered mixture may comprise by weight betweenabout 0.15% and about 3.5% TiO₂, preferably between about 0.3% and about2.7% TiO₂, and most preferably between about 0.4% and about 2.3% TiO₂.During the sintering process, which is preferably conducted at atemperature from 1300° C. to 1500° C., the TiO₂ forms a complex with thealumina to form the aluminum titanate phase.

The sintered rod may also be formulated to restrict its SiO₂ content toa specific low level (e.g., less than about 4% by weight, and preferablyno more than about 2% by weight). When the level is silica is greaterthan 4%, silica bridges the alumina crystals during the sintering stepand makes the ceramic material more fragile and breakable. By limitingthe SiO₂ content of the proppant, the sintered rod formulation ensuresoptimum strength from a high percentage of alumina (e.g., greater than92%) reinforced by the formation of aluminum titanate while at the sametime minimizing the weakening effects of SiO₂.

Iron oxide, commonly found in bauxite, can also weaken the proppant. Thesintered rod should contain no more than 10% by weight iron oxide. Wherea substantial portion of the mixture (e.g., over 80% by weight) to besintered is compromised of an alumina material that contains iron oxide(e.g., bauxite) that material should comprise iron oxide in amounts notto exceed about 10% by weight, and preferably no more than 8% by weight.This will help ensure that the sintered rod has superior strengththroughout while still being able to break into substantially uniformpieces under high closing pressure as will be further discussed below.It may also limit the production of excessive undesirable fines at highclosing pressures.

The high percentage of alumina in the sintered rods may come from anumber of bauxitic and non-bauxitic sources. For example, a high-qualitybauxite containing a high level of alumina (e.g., 85% or more) may beused as the primary source of alumina for the final composition. Inaddition to containing alumina, bauxite typically also containsadditional oxides, such as SiO₂, TiO₂, Fe₂O₃, ZrO₂, MgO. As mentionedabove, excessive amounts of certain of these oxides can weaken thesintered rod. Only bauxite that will not contribute excessive amounts ofundesirable impurities to the mixture, based upon the amount of bauxitepresent in the mixture, should be used. Suitable bauxite may come from,for example, the Weipa mine in Australia, or mines in Brazil, China, orGuinea. Since bauxite may not have a high enough alumina content toachieve the desired high alumina content in the final product, anon-bauxitic source of alumina, such as “technical grade alumina” or“pure alumina” may be used to supplement the alumina in the bauxite.Technical grade alumina contains, for example, 98%-99% alumina with onlya small amount of impurities.

In an alternative method of making a suitable sintered rod, anon-bauxitic source such as technical grade alumina may be used as theprimary source for the alumina contained in the final sintered rod. Arelatively small percentage of bauxite may be used as a supplementalsource of alumina, and may contribute a beneficial amount of TiO₂ toprovide the desired aluminum titanate in the final sintered rod. Becausethe bauxite is used in smaller amounts in this embodiment, a bauxitecontaining higher levels of impurities may be used, so long as theoverall amount of the impurities is relatively low in the final sinteredproduct.

The alumina-containing material (e.g., bauxite) may optionally be sizedusing various milling or grinding techniques, including both attritiongrinding and autogenous grinding (i.e., grinding without a grindingmedium), and may be ground by either a dry or wet grinding process. Thegrinding may be accomplished in a single step or may involve multiplegrinding steps.

Proper sizing prior to forming the sintered rods can increase thecompacity of the feed and ultimately result in a stronger proppant oranti-flowback additive. In one embodiment, a jet mill may be used toprepare a first batch of particles having a first particle sizedistribution. In a jet mill, the particles are introduced into a streamof fluid, generally air, which circulates the particles and inducescollisions between the particles. Using known techniques, the forces inthe jet mill can alter the particle size distribution of the particlesto achieve a desired distribution. For example, one may vary the type offluid used in the mill, the shape of the milling chamber, the pressureinside the mill, the number and configuration of fluid nozzles on themill, and whether there is a classifier that removes particles of adesired size while leaving others in the mill for additional milling.The exact configuration will vary based on the properties of the feedmaterial and the desired output properties. The appropriateconfiguration for a given application can be readily determined by thoseskilled in the art.

After the first batch of particles having the first particle sizedistribution is prepared, a second batch of particles may be jet milledto a second particle size distribution. The first and second batchparticle size distributions and milling conditions, and the conditionsunder which they are combined, are selected to form the desired finalparticle size distribution of the combined batches prior to sintering.Using this technique, a bi-modal particle size distribution may beobtained. The advantage of preparing a bi-modal feed is that it maycontain additional fine particles to pack between the coarser particles,leading to increased compacity and density prior to sintering. Thoseskilled in the art will appreciate that one need not stop at two batcheswith different particle size distributions, but could combine three ormore batches to achieve multi-modal particle size distributions prior tosintering. The batches of particles can be combined using any mixingtechnique known in the art for mixing dry powders, such as employingintensive mixers (e.g., Eirich mixers), which can quickly produce ahomogeneous powder blend. Using this approach, it has surprisingly beendiscovered that the resultant sintered rod achieves better compacity andcrush resistance.

In another embodiment, the alumina-containing material may optionally besized in a ball mill. Similar to jet milling multiple batches todifferent particle sizes and mixing them, ball milling may result in amulti-modal particle size distribution, which can improve the compacityof the powder. However, in contrast to a jet milling process, acceptableresults may be achieved in a single ball-milled batch of particles(i.e., there is no requirement to prepare multiple batches and mixthem). Of course, there is no technical reason to avoid combiningmultiple ball-milled batches, and one embodiment consistent with thepresent invention involves ball milling multiple batches and mixing themto form a powder with a desired multi-modal particle size distribution.In another embodiment, batches with two different particle sizedistributions can be simultaneously milled in the ball mill, resultingin a powder with a multi-modal particle size distribution.

Regarding the mechanics of the ball milling process, a ball millcontains a chamber in which the alumina-containing material and acollection of balls collide with each other to alter the material'sparticle size. The chamber and balls are typically made of metal, suchas aluminum or steel. The appropriate configuration for the ball mill(e.g., the size and weight of the metal balls, the milling time, therotation speed, etc.) can be readily determined by those skilled in theart. The ball milling process can be either a batch process or acontinuous process. Various additives may also be used to increase theyields or efficiency of the milling. The additives may act as surfacetension modifiers, which may increase the dispersion of fine particlesand reduce the chance that the particles adhere to the walls and ballmedia. Suitable additives are known to those skilled in the art, andinclude aqueous solutions of modified hydroxylated amines and cementadmixtures. In one embodiment, the ball mill is configured with an airclassifier to reintroduce coarser particles back into the mill for amore accurate and controlled milling process. Like the jet millingembodiment described above, ball milling has surprisingly beendiscovered to result in a proppant or anti-flowback additive withimproved compacity and crush resistance.

While various particle sizes and size distributions may be useful inpreparing proppants and anti-flowback additives, the pre-milledalumina-containing material may have at least 95% of its particlessmaller than 500 microns as measured by sieving or a Microtrac particlesize analyzer, and may have all of its particles smaller than 500microns. After milling, in certain embodiments the material has a d50 ofless than 10 microns, and may have a d50 of less than 5 microns, lessthan 3 microns, or even less than 1.5 microns. In one embodiment, thepowder has a d50 from 1.5 microns to 2 microns, and ratio of the d90 tothe d10 from 4 to 8. The d10, d50, and d90 may be measured using a lasermicrosizer, such as the Malvern Mastersizer 2000. The milled materialmay also have substantially all of its particles smaller than 30microns. A broad particle size distribution is preferred to a narrowone, as it is believed that the broader distribution results in anincrease of the compacity of the material and the strength of the finalsintered rod.

The sintered rod in accordance with one embodiment of the presentinvention may be prepared by first mixing the desired alumina-containingmaterials with at least one binding agent and/or solvent. The bindingagent and/or solvent is one of those well known in the industry. Somepossible binding agents include, for example, methyl cellulose,polyvinyl butyrals, emulsified acrylates, polyvinyl alcohols, polyvinylpyrrolidones, polyacrylics, starch, silicon binders, polyacrylates,silicates, polyethylene imine, lignosulfonates, alginates, etc. Somepossible solvents may include, for example, water, alcohols, ketones,aromatic compounds, hydrocarbons, etc. Other additives well known in theindustry may be added as well. For example, lubricants may be added,such as ammonium stearates, wax emulsions, elieic acid, Manhattan fishoil, stearic acid, wax, palmitic acid, linoleic acid, myristic acid, andlauric acid. Plasticizers may also be used, including polyethyleneglycol, octyl phthalates, and ethylene glycol. The mixture may then beextruded, for example, through a die, to form a rod having across-section of a desired shape, such as a substantially circular shapeor any other suitable shape. The process of extrusion may be performedusing extrusion methods known in the industry. For example, theextrusion process may be a batch process, such as by forming the rodsusing a piston press, or may be a continuous process using an extrudercontaining one or more screws. Loomis manufactures a piston press thatmay be used to batch produce the rods, while Dorst and ECT both makeextruders that contain one or more screws that may be used in thecontinuous extrusion production method. Other suitable equipment andmanufacturers will be readily ascertainable to those of skill in theart.

The extruded rod is then dried, for example, at about 50 degrees Celsiusor any other effective temperature, and reduced to the desired rodlength, as needed. Any suitable drying process known to the industry maybe used. For example, the rods may be dried using electric or gasdriers. In some embodiments, the drying process may be performed bymicrowave, with a continuous drying process being preferred. Thereduction to the desired length may be achieved through cutting using,for example, a rotating blade, a cross cutter, a strand cutter, alongitudinal cutter, a cutting mill, a beating mill, a roller, or anyother suitable reducing mechanism. In one embodiment of the inventionthe reduction to the desired length occurs as a result of the dryingprocess, forming a mixture of rods having a broad length distribution,and no cutting step is required. The length reduction occurs during thedrying as a result of the mechanical properties of the extruded rod. Inthis embodiment, the manufacturing process is simplified and lower incost as waste levels are reduced, cutting equipment need not bepurchased nor maintained, and less energy will be consumed in theprocess. In another embodiment, where a narrow length distribution isdesired, the rods having the desired length are obtained by any one ofvarious selection methods known to those skilled in the art, includingvisual or mechanical inspection, or sieving. However, classical sievingmethods tend to break the weaker rods. This is not necessarily adisadvantage, as only the stronger rods are selected by sieving. Theappropriate selection method will need to be determined on acase-by-case basis, and will depend on the goal of the selectionprocess.

The formed rod is then sintered at about 1,300 degrees Celsius to about1,700 degrees Celsius to form the sintered rod suitable for use as aproppant or anti-flowback additive. In some embodiments, the sinteringtemperature is preferably between about 1,400 degrees Celsius to about1,600 degrees Celsius. The sintering equipment may be any suitableequipment known in the industry, including, for example, rotary orvertical furnaces, or tunnel or pendular sintering equipment.

The sintered rods may optionally be coated with one or more coatings.Applying such a coating can provide various advantages, including theability to control the dispersion of fines that may be generated whenthe rods break under injection or closure pressures. Many coatings havebeen suggested in the art, with U.S. Pat. No. 5,420,174 to Dewprashadproviding the following non-exhaustive list of natural and syntheticcoatings: “natural rubber, elastomers such as butyl rubber, andpolyurethane rubber, various starches, petroleum pitch, tar, andasphalt, organic semisolid silicon polymers such as dimethyl andmethylphenyl silicones, polyhydrocarbons such as polyethylene,polyproplylene, polyisobutylene, cellulose and nitrocellulose lacquers,vinyl resins such as polyvinylacetate, phenolformaldehyde resins, ureaformaldehyde resins, acrylic ester resins such as polymerized estersresins of methyl, ethyl and butyl esters of acrylic andalpha-methylacrylic acids, epoxy resins, melamine resins, drying oils,mineral and petroleum waxes.” Additional coatings include urethaneresins, phenolic resins, epoxide phenolic resins, polyepoxide phenolicresins, novolac epoxy resins, and formaldehyde phenolic resins. One ormore of these coatings can be applied to the sintered rods using anyknown method, including both batch and on-the-fly mixing.

In one embodiment of the present invention, the sintered rod hasparallel bounding planes that are substantially circular, where thesubstantially circular planes have an average diameter of between about0.5 mm and about 2 mm. In some embodiments, the preferred diameters maybe between about 0.5 mm and about 1.5 mm. Sintered rods having a lengthof up to about 20 mm, preferably up to 10 mm, may be suitable for use asproppants or anti-flowback additives in accordance with embodiments ofthe present invention. In some embodiments, the preferred rod length maybe between about 1 mm and about 5 mm, or more preferably between about 2mm and about 4 mm.

In some embodiments, the diameter of the substantially circular planesmay correspond with diameters specified in the API standard forspherical proppants. In one embodiment, the preferred rod length may bethe naturally sustainable length limited by the drying process, forexample, the length at which the rod will not break during the dryingprocess. As discussed above, this approach can provide a useful proppantor anti-flowback additive without the step of cutting it to a particularlength, thereby simplifying and lowering the cost of the manufacturingprocess, reducing waste produced during the cutting step, simplifyinglogistics due to the reduced need to produce, store, package, and shipproppants and anti-flowback additives of different sizes, andsimplifying the planning of the fracturing job as there is no need todetermine the needed length of the proppant or anti-flowback additivefor a particular job.

Depending on the requirements for a particular fracture or proppantpack, the fracturing fluid may include either a narrow or broad lengthdistribution of the rods before closure. To create a narrow lengthdistribution, rods may be cut as described above to ensure a moreuniform length distribution. More varied lengths may exist in afracturing fluid with a broader length distribution before closure.While prior to closure a collection of sintered rods with a broad lengthdistribution may have different physical properties from a collectionhaving a narrow length distribution, after closure both collections ofsintered rods may behave similarly in the fracture. This is primarilybecause the sintered rods in accordance with an embodiment of thepresent invention have the unique ability to break into substantiallyuniform rods of smaller sizes under a closing pressure. This uniquebreaking property is discussed in more detail below. However, as a briefexample, in a pack formed from a fracturing fluid of sintered rodshaving varied lengths, the longer rods will break first under lowerclosing pressure (e.g., 2,000 psi) into intermediate and smaller rods,which may break again into smaller pieces at higher closing pressure(e.g., 5,000 psi). In this way, the pack made from fracturing fluid ofvaried length sintered rods may ultimately achieve substantially uniformlengths at certain higher closing pressures. As used herein, rods having“substantially uniform length” are rods that have the same length, plusor minus 20%. Preferably, these rods will have the same length, plus orminus 10%.

Although the sintered rods will break to the appropriate size in thefracture, it has been discovered that shorter sintered rods with anarrow length distribution may perform better than longer rods thatbreak to the same size. Thus, for some fracturing applications it may bedesirable to determine an optimum length and prepare the sintered rodaccordingly. Achieving the desired length distribution may beaccomplished by a) cutting the material after extrusion, such as byusing a rotating knife next to the extrusion die; b) cutting thematerial before or after drying, such as by using a combination of meshand knives, with the mesh being placed after the knives to sieve therods using known sieving techniques, including the use of bouncingrubber balls on top of the mesh to aid in sieving; c) cutting thematerial after sintering, such as by using an Eirich mixer or a ballmill; or d) any of the other various methods for sizing known to thoseskilled in the art.

A sintered rod having the above dimensions may have a length to widthratio (this term is also intended to encompass the length to diameterratio, if the rod has a circular cross-section) of about 1.5:1 to about20:1. In some embodiments, it may be desirable that the length to widthratio be between about 1.5:1 to about 10:1, more preferably betweenabout 1.5:1 and about 7:1. It may be further preferable to restrict thelength to width ratio from about 2:1 to about 4:1 in some embodiments.It is desirable that the sintered rod have a length to width ratio ofgreater than 1:1 because the elongated shape introduces more disorderinto the proppant pack, which increases void spaces between theproppants and in turn increases the conductivity of the proppant pack.As an example, an experiment was conducted in which equal volumes of aspherical proppant of the prior art and a rod-shaped proppant of thepresent invention, each with a bulk density of about 2.01 g/cm³ wereplaced in separate Erlenmeyer flasks. Distilled water was introducedinto each flask until the proppants were submerged in water. The watervolume needed to penetrate the voids was then measured. The volume ofwater poured into the flask represents the void volume. For thespherical proppant, 5.8 mls of water was necessary to fill the voidvolume. For the rod-shaped proppant, 10.7 mls of water wasnecessary-almost double that of the spherical proppant. This comparisondemonstrates that for the same volume of proppant, the rod-shapedproppant may have significantly more void volume than the same volume ofa spherical proppant.

In another experiment, approximately 32.9 g each of two sphericalproppants and one rod-shaped proppant consistent with the presentinvention were placed in separate Erlenmeyer flasks each filled with 50mls of distilled water. The rod-shaped proppant had a broad lengthdistribution and an average width or diameter of between about 1.1 mmand about 1.3 mm. All three of the proppants had a bulk density betweenabout 2.00 g/cm³ and about 2.01 g/cm³. The flasks were shaken slightly,but only to the extent necessary to provide a level surface on the topof the proppant. The volume level of the proppants was then measured, aswas the level of the water. From this information, the void volumewithin the proppant was calculated using the following equations:V _(void) =V _(proppants) −ΔV _(liquid)whereΔV _(liquid) =V _(liquid final) −V _(liquid initial)

The void volumes of the two spherical proppants were measured to beabout 33% and about 38%, while the void volume of the rod-shapedproppant was found to be about 50%. This further demonstrates that forthe same mass of proppant, a rod-shaped proppant consistent with thepresent invention may exhibit more void volume in the proppant pack,leading to a larger space for oil or natural gas to flow to the wellbore. The flasks were then shaken and tapped for approximately 2 minuteswith the goal of packing the proppant particles more tightly. The samelevels were measured, and the void volume in the spherical proppants didnot change in any significant manner. As expected, the void volume inthe rod-shaped proppant decreased somewhat, but it still contained avoid volume of about 44%. This packed void volume was still higher thanthat of either of the spherical proppants. Table 1 below provides thedata from these experiments.

TABLE 1 % Proppant Tapped? Weight V_(initial) V_(final) V_(proppant)V_(void) voids Spherical 1 NO 32.7 g 50 ml 60 ml 16 ml 6 ml 38%Spherical 1 YES 32.7 g 50 ml 60 ml 16 ml 6 ml 38% Spherical 2 NO 32.9 g50 ml 60 ml 15 ml 5 ml 33% Spherical 2 YES 32.9 g 50 ml 60 ml 15 ml 5 ml33% Rod-shaped NO 32.9 g 50 ml 59 ml 18 ml 9 ml 50% Rod-shaped YES 32.9g 50 ml 59 ml 16 ml 7 ml 44%

The desirable properties of sintered rods made in accordance with thepresent invention are believed to be associated, at least in part, withtheir relatively high apparent specific gravity. While “specificgravity” is known in the art to refer to the weight per unit volume of amaterial as compared to the weight per unit volume of water at a giventemperature, “apparent specific gravity” as used in this applicationrefers to the weight per unit volume of a material including only thematerial itself and its internal porosity as compared to the weight perunit volume of water. Thus, in the apparent specific gravity computationfirst the weight of the material being measured is determined. Then thevolume of the material, including only the volume of the material andits internal pores, is determined. For some materials, this step iseasily accomplished by placing the material in water and measuring thevolume of the displaced water. Indeed, under certain circumstances watermay appropriately be used for applications that compare one proppant toanother, such as in the void volume experiments described above. Forproppants of this type, however, water may permeate and fill in theinterior pores, giving inaccurate absolute results such as those desiredwhen computing apparent specific gravity. Consequently, it is necessaryto measure the displacement in mercury or some similar fluid that willnot permeate the material and fill its internal pores. The weight perunit volume measured in this manner is then compared to the weight perunit volume of water at a given temperature. The specific temperatureused in accordance with this application is room temperature, or about25 degrees Celsius.

A sintered rod prepared as described above may have an apparent specificgravity of up to about 3.98. In some embodiments, it may be desirablethat the apparent specific gravity of the sintered rods be between about3.0 and about 3.98. It may be further preferable that the apparentspecific gravity be between about 3.2 and about 3.95 in someembodiments. The specific range chosen may be based on a variety offactors including, for example, the intended use, which may involveconsiderations such as fracture depth, the type of carrier fluid, etc.The sintered rod may also have a bulk density of about 1.5 g/cm³ toabout 2.5 g/cm³. In some embodiments, the bulk density may preferably bebetween about 1.7 g/cm³ to about 2.3 g/cm³. Bulk density as used in thisapplication and understood within the art refers to the mass of aparticular volume of sintered rods divided by the volume occupied by thesintered rods where the mass has been compacted. This is sometimesreferred to as “packed” or “tapped” bulk density. The measurement methodof the “packed” or “tapped” bulk density is that set forth by theFederation of European Producers of Abrasives (FEPA) as standard number44-D. The volume used for the calculation of bulk density includes boththe space between the sintered rods and the pore spaces (both interiorand exterior) of the sintered rods.

It is known within the art that proppants having a high apparentspecific gravity and high alumina content exhibit superior crushresistance. Crush resistance as used in this application is measuredaccording to procedures promulgated by the API for measuring proppantcrush. Specifically, a certain volume of the sintered rods of aparticular dimension range (i.e., 1.1 mm-1.3 mm in diameter and 2 mm-14mm in length) is loaded into a crush cell with a floating piston. For adesired stress level, the piston presses onto the sintered rods at therequired stress level (e.g., 20,000 psi) for a set period of time (e.g.,two minutes). The weight percentage of crushed materials, for example,gathered by sieving the fines through a sieve of a desired size (e.g.,less than about 1 mm), is measured.

Results of tests using API crush resistance procedures indicate thesintered rods consistent with the present invention exhibit high crushresistance up to 20,000 psi. At 10,000 psi only between about 5% byweight and about 9% by weight were crushed. At 15,000 psi between about9% by weight and about 19% by weight were crushed. When the optionalmilling step is used, it is believed that in some embodiments only about7% to about 15% of the particles may be crushed at 15,000 psi, in otherembodiments only about 7% to about 13% may be crushed, still others mayhave only about 8% to about 12% crushed, and in other embodiments onlyabout 9% to about 11% of the particles may be crushed, with allpercentages being given by weight. For example, in a sample of sinteredrods consistent with one embodiment of the invention, only about 12% byweight were crushed at 15,000 psi. The variation in the crush resistanceat a given pressure is due, at least in part, to variations in thelengths of the rods, the diameters of the rods, the feed material, anyimpurities in the feed, the sintering temperature, and the sinteringtime.

Because crush resistance alone is generally insufficient to illustratethe potential conductivity that is essential to a proppant, aconductivity test according to the API Recommended Practice 61 formeasuring conductivity was also conducted. In a particular test, aquantity of sintered rods in accordance with one embodiment of thepresent invention was placed and leveled in a test cell between Ohiosandstone rocks. Ohio sandstone has a static elastic modulus ofapproximately 4 million psi and a permeability of 0.1 mD. Heated steelplates provided the desired temperature simulation for the test. Athermocouple was inserted into the middle portion of the rod pack torecord the temperature. A servo-controlled loading ram provided aclosing pressure on the proppant between the Ohio sandstone rocks. Thetest cell was initially set at 80° F. and 1,000 psi. The cell was thenheated to 250° F. and held for 4 hours before the stress was increasedto 2,000 psi over 10 minutes. After 50 hours at 2,000 psi, measurementswere made, and then the stress level was raised to 3,000 psi. The sameprocedures were applied and subsequent measurements were made at 5,000psi, 7,500 psi, and 10,000 psi over a total of 254 hours.

Measurements were taken of the pressure drop in the middle of thesintered rod pack to enable calculation of the permeability at aparticular stress condition according to Darcy's Law. Specifically,permeability is part of the proportionality constant in Darcy's Law,which relates flow rate and fluid physical properties (e.g., viscosity)to the stress level applied to a pack of sintered rods. Permeability isa property specifically relating to a pack of sintered rods, not thefluid. Conductivity, on the other hand, describes the ease with whichfluid moves through pore spaces in a pack of sintered rods. Conductivitydepends on the intrinsic permeability of a sintered rod pack as well asthe degree of saturation. In particular, conductivity expresses theamount of water that will flow through a cross-sectional area of asintered rod pack under the desired stress level.

Specifically, to measure conductivity, a 70 mbar full range differentialpressure transducer was started. When the differential pressure appearedto be stable, a tared volumetric cylinder was placed at the outlet and astopwatch was started. The output from the differential pressuretransducer was fed to a data collector, which recorded the output everysecond. Fluid was collected for 5 to 10 minutes and then the flow ratewas determined by weighing the collected effluent. The mean value of thedifferential pressure was retrieved from a multi-meter, as were the peakhigh and low values. If the difference between the high and low valueswas greater than 5% of the mean, the data was disregarded. Temperaturewas recorded at the start and end of the flow test period. Viscosity ofthe fluid was obtained using the measured temperature and viscositytables. At least three permeability determinations were made at eachstage using Darcy's Law. The standard deviation of the determinedpermeabilities had to be less than 1% of the mean value before the testwas accepted.

The following table summarizes the results of the above conductivitytest conducted on sintered rods consistent with the present invention,as well as high strength and intermediate strength spherical particles.The rods were between about 0.9 mm and 1.1 mm in diameter, and had anarrow length distribution centered at 10 mm.

TABLE 2 Conductivity High Strength Intermediate Pressure (psi) RodsSpherical Strength Spherical 5000 31875 6048 5487 7500 11405 4293 358910000 4390 2651 2113 12500 751 1746 1314 15000 207 1181 936 All measuresexcept pressure are in mD-ft.

When the optional milling step is employed, the conductivity of the rodsincreases to about 5200 mD-ft at 10,000 psi and about 3600 mD-ft at12,500 psi. The surprisingly superior conductivity and permeability ofthe rod-shaped proppants at high closure pressure as compared tospherical proppants that are currently being used in the industry wasfound to be largely attributable to the proppant's unique rod shape andits unexpected breaking behavior under closing pressure. Particularly,unlike a sphere, which has a single load bearing point at which theclosing pressure converges, often leading to crushing, a rod has a muchbroader area of contact in a multi-layered pack under pressure, allowingit to distribute the pressure more evenly and thereby reducing crushingand embedment at comparable closing pressures.

It is known that crushing of the current spherical proppants leads tothe creation of fines. Essentially the spheres break under pressure intovery minute, dust-like pieces that have a tendency to create denselypacked fine layers that significantly reduce both permeability andconductivity. Additionally, the fines tend to have sharp edges, whichwhen in contact with surrounding intact spheres, concentrate thecompression forces onto other spheres at the sharp contact points andcontribute to the destruction of the surrounding spheres in the proppantpack.

The sintered rods, besides being more resistant to crushing undercomparable closing pressures due to their unique shape, also exhibit thesurprising property of being able to break into generally uniform sizedsmaller rods when breakage does occur. This behavior is in contrast tothe failure of spherical particles, described above, which typicallydisintegrate when they fail and create a large amount of fines. Insteadof creating dust-like fines, the rod-shaped proppants break into smallerrod-shaped proppants. The breaking behavior of the sintered rods isattributable, at least in part, to the specific composition of a largeamount of alumina with a small amount of other synergistic oxides in thesintered rod formulation. For example, a small percentage of TiO₂ in thesintered rod composition, preferably contributed by bauxite, allows forthe formation of aluminum titanate (Al₂TiO₅) during the sinteringprocess, which provides extra strength to the sintered rod proppant oranti-flowback additive. In one embodiment, the sintered rod may containbetween about 0.2% and about 4% aluminum titanate by weight, preferablybetween about 0.5% and about 3% by weight, and more preferably betweenabout 1% and about 2.5% by weight. In some embodiments the bauxitebefore sintering may comprise by weight between about 0.5% and about 4%TiO₂, preferably between about 1% and about 3% TiO₂, and more preferablybetween about 2% and about 3% TiO₂.

The rods also maintain their unique rod shape as they break into smallerrods, thereby maintaining their efficacy as a proppant. In oneexperiment, two collections of 100 g of sintered rods, one having abroad length distribution and the other having a narrow one, were testedaccording to API Procedure 60 at 22,000 psi. As used in thisapplication, a narrow length distribution is one where at least about60% of the rods have lengths within about 1 mm of the mean. All otherdistributions are considered broad. After the experiment, the sinteredrods of both sizes were examined and found to have reached a very narrowlength distribution centered around 4 mm. Even at this high pressurenumerous rods were still intact.

The manner in which the sintered rods break has a number of advantages.The smaller rods do not behave like fines that settle into dense packsbetween still-intact spherical proppants. Thus, there is little to noreduction in conductivity or destruction of neighboring proppants asoccurs with fines in spherical proppant packs. It is also believed thatthe smaller rod pieces that result from breaking of a larger sinteredrod exhibit the same or similar beneficial properties as the largersintered rod. The smaller rods remain superior in their load carryingcapability and resistance to embedment. Moreover, to the extent finesare generated, they are believed to be less destructive to the proppantpack than the fines generated when other proppants, such as sphericalproppants, break down. This further maintains permeability andconductivity. In view of these advantages, a pack of sintered rods maytherefore exhibit superior longevity, conductivity, and permeabilityover a pack of sintered spheres under similarly high closure pressure,even when the closing pressure causes breakage of the sintered rods. Asdiscussed above, in some applications better performance may be achievedby using shorter rods with a narrow length distribution.

It is also observed that the sintered rod reduces the non-Darcy floweffect (a characterization of fluid flow that accounts for theturbulence generated as the oil or natural gas flows through theproppant pack). Non-Darcy flow reduces well production significantly andstrips the deposited proppants from the fracture, causing them to flowback to the well bore with the natural gas or oil. In particular, thenon-Darcy flow effect is mainly experienced in high flow-rate gas andvolatile oil wells. The effect arises from the fact that fluid flow nearthe well bore has a turbulence component due to a significant pressuredrop along the fracture and the convergence of flow at the well bore,which results in high flow velocities. This effect is particularlysignificant in natural gas wells due to the highly expandable and lessviscous nature of natural gas. The non-Darcy flow effect is expressedas:dp/dl=μpv/k+βρv2where p is the pressure drop in the fracture, l is the length of thefracture, μ is the viscosity of the gas, v is the velocity of the gas, kis the permeability of the fracture, β is the turbulence coefficient inthe fracture, and ρ is the density of the natural gas/oil.

A comparison was performed with regard to three different possibleproppant shapes to determine the effect of shape on the turbulencecoefficient β. It was found that an elongated shape, such as thesintered rod of the present invention, is associated with a much reducedβ as compared to a spherical or irregular shape. Therefore, rod-shapedproppants would be subject to less stripping due to the non-Darcy floweffect and result in less proppant flowing back to the well bore.

Reducing flow back to the well has a number of advantages. For example,less flowback reduces the abrasive wear on expensive well equipment,reduces the cost of clean up, and ensures that more of the proppantstays in the pack, providing a longer useful life for the well and abetter return on investment.

Although rod-shaped proppants may be used by themselves in a fracture,they may have additional utility when used in conjunction with anothertype of proppant, such as a spherical proppant. A mixture containing 10%of a rod-shaped proppant consistent with the present invention (having adiameter of between about 1.1 mm and 1.3 mm and length of about 10 mm toabout 20 mm) and 90% of a spherical proppant (having a diameter of 0.7mm) was tested according to API test 60 to determine the effect of thecombination under pressure. At 15,000 psi, the rods were smaller butwere still present in rod-shaped form (i.e., they cracked into smallerrod-shaped proppants rather than disintegrating into fines).Surprisingly, many of the rods remained relatively long, up to 15 to 17mm.

In view of the above, sintered rods in accordance with the presentinvention possess a unique combination of properties that make them anexcellent proppant or anti-flowback additive. Specifically, the highalumina content of the sintered rod ensures superior crush resistance,permeability, and conductivity at high closure pressures. Moreover, theproppant's unique shape enhances crush resistance, permeability, andconductivity by allowing even distribution of pressure throughout theproppant pack. In addition, the proppant's unique breaking behaviorprevents deterioration of the pack and lowers the reduction in thepack's efficiency as compared to spherical proppants. The unique rodshape has the added benefit of reducing the non-Darcy flow effect in thewell, thereby minimizing equipment wear and tear, maintaining consistentproduction of gas or oil, and reducing the cost involved in clean up ofthe flowback. When used in combination with other types of proppants,the presence of the rod-shaped proppant consistent with the presentinvention provides the unique advantages of increasing the void volume,decreasing proppant flowback, reducing the amount of fines generated athigh pressures, and increasing the strength of intermediate and highstrength proppants. Consequently, the rod-shaped material in accordancewith the present invention may be used separately as a proppant, as aproppant in combination with other proppants, or as an anti-flowbackadditive when mixed in certain ratios with other proppants.

The preceding description is merely exemplary of various embodiments ofthe present invention. Those skilled in the art will recognize thatvarious modifications may be made to the disclosed embodiments thatwould still be within the scope of the invention. For example, it isenvisioned that sintered rod-shaped proppants or anti-flowback additivesmay contain an alumina content from about 40% to about 80% by weight, ormay be formed using kaolin or bauxitic kaolin as a component, inaddition to those listed above. The scope of the invention is intendedto be limited only by the appended claims.

What is claimed is:
 1. A method for making a proppant or antiflowbackadditive, the method comprising: mixing a ceramic precursor and at leastone binding agent to form an extrudable mixture; extruding theextrudable mixture to form a plurality of extruded rods; reducing thelength of the extruded rods by at least partially drying the rods in amanner that results in breakage of the extruded rods to form driedextruded rods of reduced length; and sintering the dried extruded rodsof reduced length to obtain the proppant or antiflowback additive. 2.The method of claim 1, wherein the length of the extruded rods is notreduced by cutting.
 3. The method of claim 1, further comprisingfractionating the proppant or antiflowback additive to obtain a subsetof rods having a desired range of lengths.
 4. The method of claim 3,wherein the fractionating comprises sieving.
 5. The method of claim 1,wherein the proppant or antiflowback additive has an average length towidth ratio ranging from about 1.5:1 to about 7:1.
 6. The method ofclaim 1, wherein the proppant or antiflowback additive has a packed voidvolume of greater than 40%.
 7. The method of claim 1, wherein theproppant or antiflowback additive has a specific gravity ranging from3.0 to 3.98 and a conductivity of greater than 3,000 mD-ft at 10,000 psias measured under API Recommended Practice
 61. 8. The method of claim 1,wherein said at least one binding agent is selected from the groupconsisting of methyl cellulose, polyvinyl butyrals, emulsifiedacrylates, polyvinyl alcohols, polyvinyl pyrrolidones, polyacrylics,starch, silicon binders, polyacrylates, silicates, polyethylene imine,lignosulfonates, and alginates.
 9. The method of claim 1, furthercomprising adding at least one solvent during the mixing.
 10. The methodof claim 9, wherein the at least one solvent is selected from water,alcohols, ketones, aromatic compounds, and hydrocarbons.
 11. The methodof claim 1, wherein the ceramic precursor comprises at least 80% aluminaand 0.15% to 3.5% TiO₂.
 12. The method of claim 1, wherein the proppantor antiflowback additive comprises between about 0.2% by weight andabout 4% by weight aluminum titanate.
 13. The method of claim 1, whereinfrom about 7% by weight and about 13% by weight of the proppant orantiflowback additive is crushed at 15,000 psi.
 14. The method of claim1, wherein from about 9% by weight to about 19% by weight of theproppant or antiflowback additive is crushed at 15,000 psi.
 15. Themethod of claim 1, wherein the extruded rods are sintered at atemperature ranging from about 1,300° C. to about 1,700° C.
 16. Themethod of claim 1, wherein the extruded rods have an average lengthranging from about 1 mm to about 5 mm.
 17. The method of claim 1,wherein the extruded rods have an average length ranging from about 2 mmto about 4 mm.
 18. The method of claim 1, further comprising coating thesintered rods with a coating agent selected from natural rubber, butylrubber, polyurethane rubber, starches, petroleum pitch, tar, asphalt,organic semisolid silicon polymers such as dimethyl and methylphenylsilicones, polyhydrocarbons such as polyethylene, polyproplylene,polyisobutylene, cellulose and nitrocellulose lacquers, vinyl resinssuch as polyvinylacetate, phenolformaldehyde resins, urea formaldehyderesins, acrylic ester resins such as polymerized esters resins ofmethyl, ethyl and butyl esters of acrylic and alpha-methylacrylic acids,epoxy resins, melamine resins, drying oils, and mineral and petroleumwaxes.
 19. The method of claim 1, further comprising cutting ormechanically breaking the extruded rods to achieve a narrower lengthdistribution.